1. Field of Invention
This invention relates to a microwave component with a periodic lattice structure to achieve filtering and switching of microwave signals.
2. Brief Description of the Prior Arts
The term “Photonic Band Gap” (PBG) was initially used in optical regime where a strong reflection in a certain range of frequency is observed. Such reflection is caused by periodic changes of dielectric layers with different indices of refraction. Since the propagation of light is prohibited in such a range of frequency, it is referred to as the “band-gap” [E. Yablonovitch, Phys. Rev. Lett., 58, pp. 2059-2062, 1987]. This remarkable property inspires many researchers to put great efforts into the development of PBG structures in microwave and millimeter-wave components [Yongxi Qian and T. Itoh, 1999 IEEE MTT-S International, Microwave Symposium Digest, Vol. 4, pp. 13-19, June 1999]. Interests have been paid to microwave PBG structures because of their extraordinary features such as prohibiting electromagnetic waves to travel at frequencies within the PBG. In addition, the PBG structure is an attractive design because it can be integrated with microstrip transmission lines not only to provide better performance, but also to reduce the size and cost of the microwave and millimeter-wave components.
A good PBG design requires a large attenuation in the stop band, controllable bandstop width and controllable central bandstop frequency. Several designs of PBG with different lattice pattern and perforations embedded in either the ground plane or the dielectric substrate of the microstrip transmission line structure have been reported to have bandstop characteristics [V. Radisic, Y. Qian, and T. Itoh, IEEE, Microwave and Guided Wave Letters, Vol. 8, Issue 1, pp. 13-14, January 1998] [Fei-Ran Yang, Kuang-Ping Ma, Yongxi Qian and T. Itoh, IEEE, Microwave Theory and Techniques, Vol. 47, Issue 8, pp. 1509-1514, August 1999]. A lattice pattern consists of more than one perforations and it may be one-or two-dimensional. For example, a PBG structure 1 shown in FIG. 1 has a one-dimensional one-row lattice pattern, which consists of four rectangular perforations (5, 6, 7, 8) while another PBG structure 14 shown in FIG. 3 has a two-dimensional rectangular lattice pattern 19, which consists of fifteen circular perforations 18. PBG structures for microwave frequencies can be categorized into three groups: dielectric-based PBG, planar PBG, and uniplanar-compact PBG (UC-PBG).
The dielectric-based PBG structures are structures where the lattice pattern, which consists of perforations, is located inside of the dielectric substrate. Therefore, the propagating microwaves traveling in such structures come across periodic change of dielectric permittivity and the bandstop is effectively created. In addition to rectangular lattice pattern, other lattice patterns such as honeycomb and triangular ones with various types of perforations such as circular perforations and square perforations may be adopted in the dielectric-based PBG structures. The attenuation value of the bandstop is proportional to the perforation size (For example, each of the perforations showed in FIG. 1 has a size or area of d1×l1 and the ones in FIG. 3 have a size or area of πr12). Since the traveling electromagnetic waves are localized around the microstrip transmission line, hence the perforations have to be directly under the line to have effective bandstop characteristics. These dielectric-based PBG structures can be incorporated with power amplifiers for harmonic tuning to increase the power-added efficiency. Moreover, the effect of bandstop can be cascaded serially to create a wide bandstop width. However, the drawback of the dielectric-based PBG structures is that drilling of the dielectric substrate is required to create the perforations.
Planar PBG structures do not require perforation drilling in the dielectric substrate. The lattice pattern is located in the ground plane of the microstrip transmission line where the perforations can be etched easily. A top view of a planar PBG structure 1 is shown in FIG. 1(a) and a cross-sectional view of structure 1 along A-A′ is also given in FIG. 1(b), where on the front surface of a dielectric substrate 2 with a thickness of h1, a microstrip line 3 having a width w1 and a thickness t1 is deposited. A ground plane 4 with a thickness of t2 is deposited at the back surface of the dielectric substrate 2 where four rectangular perforations 5, 6, 7, 8 are etched inside of the ground plane 4 to form a one-row lattice pattern. Each of the perforations 5, 6, 7, 8 has a length l1, a width d1 and a distance between adjacent perforations of a1. It is noted that the microstrip line 3 is located substantially at the center of the perforations 5, 6, 7, 8 (indicated by d1/2 from the edge of the perforations). The purpose of the lattice pattern shown in FIG. 1 is to generate interferences with the traveling electromagnetic waves so that bandstop characteristics can be created.
The characteristics of a microwave component are often given in plots of S-parameters. A typical graph of forward transmission coefficient S21 versus frequency for a bandstop filter is given in FIG. 2. Here it is seen that in the low frequency region (less than 10 GHz), the forward transmission coefficient (S21) of this filter is about 0 dB. The transmission coefficient decreases as the frequency is increased and reaches a minimum at about 16 GHz. With a further increase in the frequency, the coefficient increases and reaches 0 dB at about 20 GHz. The S21 characteristics of the filter in FIG. 2 are thus divided into three regions: a lower bandpass region 9 at frequencies from 0 to 10 GHz, a bandstop region 10 from 10 to 20 GHz and an upper bandpass region 11 from 20 to 25 GHz. Here it is noted that the maximum attenuation 12 is −25 dB whereas the central bandstop frequency 13 of the bandstop region 10 is 15 GHz and the bandstop width is 10 GHz (from 10 to 20 GHz).
It is important to point out that the dimensions of perforations and the arrangement of lattice pattern determine the bandstop characteristics [J. Wu, I. Shih, S. N. Qiu, C. X. Qiu, P. Maltais, D. Gratton, 2nd CanSmart Workshop, Smart Materials and Structures, pp. 171 -179, October 2002]. When the number of perforations is increased, the absolute value of maximum attenuation increases. The central bandstop frequency of the PBG structure is related to the period distance (a1, in FIG. 1) as follows:
      a    1    =                    λ        g            2        =          c              2        ⁢        f        ⁢                                            ɛ              eff                        ⁡                          (              f              )                                                          a1=Period distance of the perforations                    λg=Guided wavelength                        c=Velocity of propagating wave in free-space                    f=Propagating frequency                        εeff(f)=Frequency-dependent effective permittivity        
It should be mentioned that PBG structures with different lattice pattern and perforations can be constructed. FIG. 3 shows a planer PBG structure 14 built on a dielectric substrate 15 with a microstrip line 16 of width w2 and a ground plane 17. The microstrip line 16 is deposited on the front surface of the dielectric substrate 15 and the ground plane 17 containing the lattice pattern 18 is deposited on the back surface of the dielectric substrate 15. This PBG structure 14 has a rectangular lattice pattern 18, which consists of fifteen (3×5) circular perforations 19, fabricated inside of the ground plane 17 to create a bandstop phenomenon [V. Radisic, Y. Qian, R. Coccioli, and T. Itoh, IEEE, Microwave and Guided Wave Letters, Vol. 8, Issue 2, pp. 69-71, February 1998] [Taesun Kim, Chulhun Seo, IEEE, Microwave and Guided Wave Letters, Vol. 10, Issue 1, pp. 13-15, January 2000]. The radius of each circular perforation 19 is r1 and the distance between adjacent perforations is a2, which is also called the period distance of the lattice pattern 18. It should be noted that the central bandstop frequency of this planar PBG structure 14 is depended on the period distance (a2) and the size of the perforations 19, given by r1, which is the radius of the circular perforations 19. Therefore, the planar PBG structure 14 can be designed with desired bandstop characteristics and applied in microwave and millimeter-wave components.
A UC-PBG structure is similar to a planar PBG structure because both types of structures have lattice patterns created in the ground plane. However, UC-PBG structures can be made more compact in size without losing the ability to create the bandstop effect. The size of UC-PBG structure can be significantly smaller than the planar PBG structure because of its unique design of the lattice pattern, which consists of metal pads and connecting branches. FIG. 4 shows a typical UC-PBG structure 20 with a microstrip line 21 of a width of w3 deposited on the front surface of a dielectric substrate 22, a lattice pattern implanted inside of the ground plane 23, which is deposited on the back surface of the dielectric substrate 22. The lattice pattern consists of several unit cells 24, which are made of metal pads 25 and metal branches 26. The distance between adjacent unit cells is α3. The metal branches 26 and the gap spaces 26′ between each unit cell 24 introduce series inductance and shunt capacitance respectively. Thus, the propagation constant is much larger than the conventional microstrip line structure due to these two additional components. Again, the central bandstop frequency is dependent on the period distance (α3).
For microwave applications, it is advantageous to have PBG structures with tunable microwave characteristics. Some computation work has been reported on a PBG structure assuming optical excitation [D. Cadman, D. Hayes, R. Miles, and R. Kelsall, High Frequency Postgraduate Student Colloquium, pp. 110-115, September 2000.]. The PBG structure 27 considered by Cadman et al is shown in FIG. 5, where circular perforations 28 are assumed to be inside of a ground plane 29, which is deposited on the back surface of a photoconductive substrate 30 made of silicon (Si). A microstrip line 31 is deposited on the front surface of the photoconductive Si substrate 30 to have a width w4. The circular perforations 28 have a radius of r2 and a distance between adjacent perforations of a4. The central bandstop frequency is dependent on the period distance (a4) and the attenuation is depended on the radius of circular perforations, r2. As the light is shined on the PBG structure 27 where the perforations 28 are located, electron-hole pairs are generated and the conductivity of the photoconductive Si substrate 30 that is exposed to the light is increased. Thus, an effectively continuous ground plane (without the perforations) is formed and the structure behaves like an ordinary microstrip transmission line (Refers to “bandstop-off” state shown in FIG. 2). Without the illumination, the conductivity of the photoconductive Si substrate 30 is low and the PBG structure 27 produces a bandstop effect (Refers to “bandstop-on” state shown in FIG. 2). There are certain drawbacks in the PBG structure 27. In order to achieve microwave switching, the intensity of light needed is high, which will cause most part of the conductive Si substrate 30 to be conducting. Hence, when the PBG structure 27 is illuminated, the resistance between the transmission line 31 and the ground plane 29 will be substantially decreased, causing un-wanted losses of microwave signals or rendering the PBG structure 27 to be useless. Hence, in addition to the high light intensity requirement, it may not be possible to “switch off” the bandstop effect of the PBG structure 27.
From the above description, it is evident that tunable or switchable PBG structures with low losses, high isolation and low operating power for tuning or switching will be very useful for microwave components and units.